The present invention relates to a gasification system and process for converting generally solid feedstock such as carbonaceous material into desirable gaseous products such as synthesis gas. Gasification processes are widely used to convert solid or liquid feedstocks such as coal, petroleum coke and petroleum residue into synthesis gas. Synthesis gas (or syngas) comprises mainly hydrogen gas and carbon monoxide, and is used both for power generation, as well as a feedstock for producing chemicals including methanol, ammonia, synthetic natural gas and synthetic transportation fuels.
The three basic types of processes that exist for the gasification of carbonaceous materials include: (1) fixed-bed gasification, (2) fluidized-bed gasification and (3) suspension/entrainment gasification. The majority of advanced gasification processes in use today utilize suspension or entrainment gasification. The present invention relates to an entrained gasification system and process for gasifying carbonaceous materials.
Suspension/entrainment gasification may be further defined as having either one or two stages for feedstock entry. All gasification reactor designs include a first reactor stage where carbonaceous feedstock and an oxidant are fed and partially combusted to create products comprising synthesis gas and slag. The resulting raw syngas usually contains only a residual level of volatile tars. The raw syngas exiting these single stage gasification processes is often at a temperature exceeding 2500° F., and requires that much of the sensible heat in the gas be removed prior to further clean-up or use. This typically is achieved by heat exchange in a high temperature heat recovery unit (HTHRU), by quenching with cool syngas, or by direct water quenching.
In certain two-stage gasification processes, an alternative, chemical quench is utilized to recover heat from the raw syngas, while simultaneously increasing the heating value of the syngas. This chemical quench includes a second gasification stage, where a second portion of carbonaceous feedstock is reacted in a low oxygen environment with the raw syngas mixture created in the first stage. The heat generated in the first stage drives endothermic chemical reactions in the second stage to generate additional syngas from this second portion of feedstock. Feeding the second portion of feedstock to the reactor as a slurry with water also assists in increasing the heating value of the product syngas, while serving to further quench the temperature of the raw syngas, thereby decreasing the amount of heat that must be later recovered in a high temperature heat recovery unit (HTHRU). A disadvantage of such two-stage gasification processes is that they often result in a higher level of volatile tar in the raw syngas versus one-stage processes. These tars must be removed to prevent harmful emissions to the environment, as well as fouling of downstream syngas processing equipment (including the HTHRU). One solution that has been utilized to eliminate such tars is to first direct the raw syngas through a residence vessel that allows sufficient high temperature residence time for the tars to thermally “crack” into smaller hydrocarbon compounds.
As mentioned above, to increase overall gasification process efficiency, sensible heat in the raw syngas produced in both single-stage and two-stage gasification processes is often recovered using one or more HTHRU. However, these units are expensive to build and install and require regular maintenance to manage fouling. To reduce costs, some manufacturers of single-stage gasification systems have alternatively implemented a complete water quench of the raw syngas produced in their single-stage reactor as a means to allow elimination of the HTHRU. However implementing an immediate water quench is precluded in two-stage gasification systems by the need to remove the residual tars created during pyrolysis of the slurrified feedstock added in the second-stage. Thermal cracking of these tars requires temperatures above about 1500° F. Additionally, fully water quenching the raw syngas creates contaminated “black water” that requires expensive clean-up processes to prevent pollution of the environment, and often necessitates that the quenched raw syngas be reheated prior to further clean-up and water-gas shifting the syngas to increase hydrogen content.
What is needed are improvements to two-stage gasification systems and processes that can simultaneously: 1) reduce the cost required to build and install such systems, while maintaining the efficiency, thereby reducing operational expenditure, 2) maintain near-zero levels of tar in the produced syngas, 3) moisturize syngas in preparation for water-gas shift without resorting to using process steam, expensive boiler feed water, or complete immersion quenching, and 4) increase overall system reliability, thereby decreasing downtime.